[python] Re: Lean Steer Python/Hipparion Trike

From: Henry Thomas <whpthomas@xxxxxxxxx>

To: python@xxxxxxxxxxxxx

Date: Fri, 05 Jan 2007 21:13:33 +1000

Well I must confess I am very glad I asked for your input, in reviewing
some of the geometry in my simulation, I found a few errors. I spend
most of the day going over it all again. It is very exacting and hard to
get your head around at times. I haven't updated the design yet, but I
can provide some specific details about the geometry to answer these
questions.

George Durbridge wrote:

I am still coming to grips with your design, which is extraordinarily
interesting and has a lot of good points, but I have three queries
already. Have you, btw, seen Paul Sims' and Bob Horn's leaning trike
designs? If not, I will try to find links.

First, if the rocker and swing arms need ends as strong as the 5mm plate
lugs you specify, and on Dirk's comments they well may, is it really
suitable to weld the lugs onto 1.5mm plate endcaps? It will be a
difficult weld, and even with a good weld the lug will be prone to tear
out of the endcap. Why not make the tube element of each arm 30mm
longer at each end and let in a transverse tube to take the bolt, like
your rear axle arrangement?

Yes after reading dirks remarks I had a similar concern. I plan to beef
it up a bit. I may also add an elastomer to the design to dampen
excessive moments -- according to Bicycle Science, these are the things
that actually cause frame failures.

Second, is it possible to achieve both self-restoring geometry (i.e. the
centre of gravity is raised by leaning) and enhanced cornering by
leaning (i.e. the CoG is lowered by leaning) in one design? I
appreciate that when the trike leans the rider rises relative to the
frame, and the frame sinks relative to the ground, but at any one angle,
surely one of those effects has to prevail, giving an overall reduction
of CoG height (better cornering, but no restoring action) or an overall
increase of CoG height (other way about).

I have attached two screen captures of the geometry simulation. I assume
that the CoG is @500mm above the ground and the seat hight is 240mm. The
first image shows the trike when it is centered. I have added
annotations and circled the two driven dimensions that reflect these
values. In the second image, the trike is leaning 25 degrees and the CoG
is almost unchanged, yet the seat height has raised 17.8mm.

So I guess it is possible, even though I wouldn't believe it either
unless I saw it with my own eyes.

Third, does the CoG move outward as the trike leans? That is, does the
CoG move horizontally toward the outside of the corner, in addition to
any vertical movement? In the photo of the earlier design, it looks as
if it does, and any self-restoring lean mechanism I have come up with
has the same effect. It is an effect I would try to avoid, as the limit
on ground loops is the angle with the horizontal made by a line from the
CoG to the contact patch of the outer wheel: the lower this angle the
better, but the angle increases as the CoG moves horizontally outwards.

I have drawn the lines you describe into the geometry for reference. The
CoG does move outwards but only marginally, and simply applying a bit of
extra body lean will bring you back to true center. Yet at the same time
leaning back just a bit the other way will cause the trike to move back
to center. On the old Prototype A, it had the effect of allowing you to
stop without taking your feet off the pedals, yet once you were riding,
it was like a normal recumbent, you would lean into corners etc...

Whether the new design actually works as expected is anyone's guess, we
will just have to wait and see.